1. Field of Invention
The present invention relates to MEMS-based optical scanner systems for endoscopic optical imaging of tissues and other biomedical specimens.
2. Discussion of the Background
The endoscopic system for the observation of the interior of a patient's body typically includes an elongated insertion tube ending with an optical window and connected to an external unit. The external unit is also known as the optical engine and contains the light source or sources, the detector and the processing unit. A light beam from the light source is delivered to and from the sample typically by an optical fiber. A variety of high resolution optical imaging modalities can be used to augment the conventional white light endoscopic observation including Optical Coherence Tomography (OCT), single and multi-photon fluorescence, confocal fluorescence, Raman etc. In the case of high resolution imaging, the light beam 2 needs to be focused and scanned on the sample 4 by a scanning mechanism 6 located at the distal end of the endoscope as shown in FIG. 1A.
Optical Coherence Tomography (OCT), an emerging optical technology analogous to ultrasound, is an interferometric technique providing microscopic tomographic sectioning of biological samples with mm-range penetration capability in tissue. By measuring singly backscattered light as a function of depth, OCT provides subsurface imaging with high spatial resolution (˜10 μm) within a depth range of 1 to 4 mm in vivo with no need for fluorescent labeling of the tissue under investigation. OCT has been demonstrated to provide accurate sub-surface imaging of highly scattering tissues in the mucosa of gastrointestinal, respiratory, and urogenital tracts as well as in the oral cavity and in the endothelium of vascular tissue. The imaging capability of OCT permits accurate non-invasive diagnosis and staging of cancers and other pathologies that heretofore have been difficult to diagnose at an early stage because of the relatively poor visualization available with white light endoscopy and ultrasonic techniques.
OCT relies on interferometry using a low-coherence light source to achieve imaging depth having an axial resolution that is inversely proportional to the source bandwidth. Sources that have been used to implement OCT are, for example, superluminescent diodes, pulsed lasers and swept-wavelength lasers. A typical source center wavelength is 1310 nm and typical source bandwidth ranges from 70 nm to 100 nm. In the OCT apparatus shown in FIG. 1B, the source light 8 is coupled into an optical fiber 10 and then split, for example by a fiber coupler 16, so that the light beam goes both through a reference arm 20 and also through a probe arm 22 to the tissue sample 4 to be imaged. The light from the reference arm 20 is reflected from a reference mirror (not shown), and the light from probe arm 22 is reflected from the tissue sample 4. Both reflections are directed back through the fiber coupler 16, and detector 12 detects interference of the two reflected beams, according to a difference in path lengths. Detection of back reflected signal has been demonstrated using different approaches, such as time domain OCT and Fourier domain OCT. In time-domain OCT the reference mirror is physically moved such that the reference path length changes, so that light reflections from different depths of tissue are sampled. An alternate approach to time-domain OCT, called Fourier-Domain OCT (FDOCT), has been shown to have significant advantages in speed and signal-to-noise ratio. In the Fourier-Domain method, the reference mirror is fixed, and the Fourier Transform of the signal is performed based on either a spectrometer-based receiving unit with a line scan camera or a swept-wavelength laser source. A computer system performs the processing steps for the image acquisition, analysis and display.
For endoscopic applications of OCT, the focused illumination beam needs to be scanned across the sample in order to obtain a tomographic image which represents a line on the sample (for one dimensional scanning) or an area of the sample (for two dimensional scanning). Since the depth information is provided by the interferometer arm in OCT, a 2D beam scanning image provides a 3D volumetric image. Two probe designs have been used in endoscopic optical imaging: side-looking and forward-looking.
Side-looking endoscopes have been developed based on configurations of rotating or translating fiber and micro-optics for imaging into a side-looking geometry. Side-looking catheter OCT probes have included both rotational-scanning and linear-scanning, the former performing a circumferential scan around the probe perimeter and the latter performing a linear scan along one probe radius. A third dimension can be added by mechanically pulling the cable along the longitudinal axis; however, this procedure has limited accuracy.
One advantage of forward looking OCT probes is their ease of integration for operation with a forward-looking optical endoscope, so that an optical image is co-registered with a coherence tomography image, as both the optical image and the tomography image are obtained from the tip of the endoscope, which can be positioned by the operator against the surface of the internal organ to be investigated.
Side-looking probes have performed well in tightly restricted organs such as the esophagus. However, side-looking probes are less successful in probing the mucosa of larger or hollow organs such as the stomach, colon, and bladder. In particular, it is quite difficult to position side-looking probes to accurately image polyps in the colon, which is one potentially promising application of EOCT. Forward-looking probes are also advantageous over side-looking for applications of image-guided surgery and intravascular imaging to detect vascular defects such as vulnerable plaques.
The design of a forward-looking OCT imaging endoscope is complicated by the need to fit a two-dimensional optical beam steering system inside the endoscope. Forward-looking imaging has been achieved by scanning a single-mode optical fiber at the distal end of an endoscope with a variety of actuators; for example, piezoelectric ceramic or electroactive polymer actuators bonded to the fiber. The conventional scanning-fiber approach may exhibit drawbacks such as a requirement of a long rigid actuator component at the distal end of the probe (which limits flexibility) or the drawback of achieving large displacements at the expense of speed. The use of galvanometers or other macroscopic scanning mechanisms limits the ability to produce a forward-looking small diameter flexible probe.